Inlays, onlays and overlays: when the indirect restoration beats the direct one

The choice between direct and indirect restoration is often made on cavity size judged by eye. More precise criteria exist, and they lead to different decisions from the habitual ones.

The first criterion is the occlusal isthmus: the buccolingual width of the cavity relative to the intercuspal distance. Beyond half that distance, the tooth's resistance falls significantly and the cusps become vulnerable.

The second is the presence and thickness of the remaining walls. A wall under 2 millimetres thick at its base will not withstand masticatory load for long, which is why it should be covered rather than rebuilt.

The third is the position of the cervical margin. A deep margin, below the cemento-enamel junction, makes isolation and finishing of a direct composite difficult, whereas an indirect restoration allows it to be managed with margin elevation techniques.

Terminology distinguishes three situations. The inlay stays within the cusps; the onlay covers one or more cusps; the overlay covers the entire occlusal surface. The difference is not nominal: it changes force distribution.

Cuspal coverage is what most alters prognosis. A restoration covering the cusps converts forces from wedging into compressive, and this protects the tooth rather than driving it towards fracture.

On an endodontically treated premolar with both walls thin, an overlay has a distinctly better prognosis than an inlay, and in many cases better than a crown: it preserves more tissue for the same protection.

Among materials, lithium disilicate is today the most used for inlays, combining aesthetics and strength. It requires at least 1.5 millimetres of occlusal thickness and rigorous adhesive cementation.

Laboratory composites have an elastic modulus closer to dentine and are easier to adjust and repair intraorally. They suit bruxist patients better, where a rigid ceramic transfers load to the antagonist.

Cast gold retains the longest documentation and the most reliable marginal behaviour, but aesthetic acceptance has reduced its use to almost nothing outside non-visible posterior sectors.

Adhesive cementation is where longevity is decided, and it requires rubber dam. A ceramic inlay cemented in a contaminated field has a worse prognosis than a well-executed direct composite.

Cervical margin elevation with composite, performed before the impression, brings a deep margin into a manageable position. It is a documented technique allowing treatment of cavities that would otherwise require surgical lengthening.

In summary: move to indirect when the isthmus exceeds half the intercuspal distance or walls fall below 2 millimetres, cover the cusp when it is weakened, and choose the material according to parafunction. Cavity size alone is not enough to decide.